1. Field of the Invention
This invention relates to radio antennas, and in particular to directional antennas arranged for point to point communication. Various proposals have been made for local microwave distribution systems, in which generally a central node is connected by a fixed cable (optical fibre or conventionally wired), or by other means, to other switched or packet systems, the central node acting as a distribution point from which a large number of end users can be served by microwave links.
2. Related Art
Such systems have been proposed for many years: see for example an article 29 GHz Point to Point Radio Systems for Local Distribution by S Mohamed and M Pilgrim in the British Telecommunications Technology Journal Vol2, No 1 (January 1984). Generally, each end user employs a directional antenna aimed at a corresponding antenna at the central node. In certain cases one end user's installation may act as a relay station to allow communication between the central node and a second end user which is out of range of the central node (typical range for a 40 GHz transmitter is of the order of 2 km), or does not have an unobstructed line of sight to the central node.
More recent proposals have extended this principle to develop a “mesh” system, in which only a few base stations are required and the user stations are connected to their nearest base station through one or more such relays. Such a system is illustrated in International Patent Specification WO98/27694. To provide multiple routing for packet data systems, and for sufficient robustness to the system in the event of a user station ceasing to operate, either temporarily as the result of a system failure or permanently (for example should the user no longer wish to use the service), each user station is provided with several antennas for provision of links with several neighbouring user stations. The mesh may be served by more than one base station, as shown in FIG. 1.
When a new user station is to be connected to the network, the connectivity of the mesh has to be changed to accommodate it. This requires re-alignment of the the directional antennas of some of the neighbouring stations, so that the new station can be connected into the mesh. Similarly, if a station is taken out of service, antennas on neighbouring stations may have to be redirected. It is envisaged that such redirection be carried out by the network operator remotely, rather than requiring a site visit.
One way to achieve this is disclosed in International Patent Application WO 99/65162, in which an fixed array of thirty-two directional antennas is provided. Each antenna is aligned in a different azimuthal direction. The antennas are switched on or off according to the current requirements of the mesh network. Several adjacent antennas can be used together as a phased array. This system is somewhat cumbersome as it requires space for a large number of antennas, only a few of which are in use at any one time. An alternative arrangement shown in International Patent Application WO 99/65105 uses a remotely controlled mechanically steerable antenna. This reduces the volume of the installation. However, in order to act as a relay the station must have more than one such antenna, each independently controlled. To avoid fouling each other, each antenna would have to be mounted in a volume clear of the other antennas' swept volumes. The simplest arrangement is a vertical stack of such antennas, each rotatable about a common vertical axis. However, such an arrangement is cumbersome, and its size and weight makes rooftop installation difficult. It is desirable to minimise the size of such equipment for reasons of materials costs, wind loading, simplicity of installation, and aesthetics.